by Boris Podobedov and R. Nagaoka
- Review of collective effects for low emittance rings (K. Bane)
- Impedance calculation and minimisation for low emittance rings (T. Günzel)
- Harmonic cavity and multi-bunch simulations for MAXIV (M. Klein)
- CSR-driven longitudinal instability - comparison of experimental and theoretical results (P. Kuske)
- Wakefield measurement with EOS (A.-S. Müller)
- OASIS: Self-consistent simulations for collective effects (G. Bassi)
- Review of beam ions instabilities (L. Wang)
- Measurement of transverse single bunch instability thresholds at APS (V. Sajaev)
- Effect of impedance of kicker and coatings in damping rings (E. Koukovini Platia)
- Report on the mini-workshop on heating from short bunches (G. Rehm)
- Measurements of the evolution of electron cloud with time-resolving electron detectors (J. Sikora)
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What are the limits on stored current in ultra-low emittance lattice?
- Low horizontal dispersion  low momentum compaction  short (zero current) bunch length  Lower instability thresholds, Interaction of the beam with high frequency wakes
- Smaller physical aperture  Larger machine impedance
- Spoiling of the emittance (H and V) due to IBS

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What are the main collective effects? Is IBS the limiting factor?
- Single bunch instabilities (microwave strong and weak, TMCI, headtail, bunch lengthening), and resistive-wall and beam-ion instabilities in multibunch
- Some LSs (such as APS and SOLEIL) must deliver high current/bunch mode for users
- Single bunch thresholds may not be far above the bunch current in multibunch operation (MAX
IV …)
- Yes, IBS would generally be a limiting factor for any low emittance ring
- We should explore whether going to negative momentum compaction could be beneficial for
USRs having intrinsically large natural chromaticities, but it may not be compatible with the low emittance optics
- Transverse feedback successfully mitigates RW instability in many 3 rd generation LSs (such as
SOLEIL), but its limit must nevertheless be quantitatively evaluated carefully for USRs (higher growth rates)
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Are round beams necessary to counteract IBS?
- Since for USRs, the vertical emittance would be diffraction limited over most photon energies of interest, a round beam should be beneficial in suppressing the horizontal emittance blow up

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Discussions on IBS:
- We could still have a factor of 2 of discrepancies in the IBS calculations depending upon different treatments.
- In this respect, further studies must be made to better master IBS especially in extremely low
emittance (violent) regime;
* Consistency among different approaches (theory, tracking, Fokker-Planck, …)
* Treatment of non-Gaussian distributions
* Modeling of tails (are tails important? … Heated discussions)
* 6D simulation would be necessary
* Further experimental bench-markings are very welcome (e.g. studies made at SLS)
- Active bunch lengthening is beneficial to fight against IBS
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Is IBS emittance growth an issue for Touschek lifetime as well in ultra low emittance rings?
- For rings that have Touschek lifetime increase with decreasing emittance, IBS emittance growth is an issue.
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What is the optimal RF frequency? Is it a possible parameter for optimisation?
- A lower frequency RF would have a beneficial effect of lengthening a bunch, but has the disadvantage of increasing the bunch current due to a reduced number of buckets (i.e. self compensating effects).

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Is heat load on the pipe components (and on the beamlines) a problem?
- Yes, it represents a critically important issue for any low emittance ring operating at high current (BPM accuracy, ion desorption, ID tapers, …)
- Again, active bunch lengthening should be an efficient way to mitigate the issue
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Is CSR a problem, in what conditions and how can it be mitigated, is shielding a viable option?
- For ultra-low emittance LSs, it does not seem to be a problem
- It could be a problem if operated in a low alpha mode
- For low emittance damping rings, it is a potential danger of blowing up the energy spread
(especially without shielding)
- Efforts must be continued to better understand (qualitatively & quantitatively) the instabilities driven by CSR (impedance-wise and beam dynamics-wise ((short and long range)))
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Are ions (ion trapping and fast-ions) an issue, in what conditions and how can it be mitigated?
There are several ways of mitigation (transverse feedback, multi-trains, chromaticity, coupling, …) which may be applicable.
Transverse feedback would not be efficient against emittance blow-ups
Combined effects such as observed at SOLEIL (RW, beam-induced heating, FBII, transverse feedback, …) require further studies and attentions

- What are the main sources of impedance (narrow and broad band)?
- What are the guidelines for the design of a low impedance machine?
- Are the estimates (analytical and numerical) of the impedance budget in agreement with measurements?
- How to model heating of a pipe?
- Pros and cons of circular versus flat chambers
- Could taper impedance be further optimised?
- How capable are the Geometric wake field solvers today?
- Short bunch wake … how to get them
- Layers/RW analytical versus numerical
- Roughness
- Loss factor extensions (coupled bunch)
- Heat propagations and cures
- What are the effective existing means to mitigate/cure instabilities?
- What are the main tracking codes for collective effects? Do they need further developments?
- Do they agree with measurements?
- What studies can be performed on existing rings to gain confidence towards building a low-emittance ring?
- Efficiency chromaticity shifting
- Pros and cons of Tracking / Linearised Vlasov / VFP methods
- VFP / Vlasov any extensions
- CSR instability studies extensions (impact transverse/coupled bunch/combined impedance, …)
- Microwave … a performance limiting instability
- Instability diagnostics (wake field measurement and comparison)
- Bunch by bunch profile
- Is transverse feedback reliable enough today?
- Digital or analogue?
- Technology limit

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WP coordinators:
R. Bartolini, S. Guiduici, Y. Papaphilippou
Task coordinators:
R. Nagaoka, E.Karantzoulis, E. Al-Dmour, G. Rumolo,
F. Antoniou, T. Demma, A.-S. Müller